1. Field of the Invention
The present invention relates to a method for storing natural gas and to an improved adsorbent for use in this method.
2. Description of Related Art
A method for storing natural gas which comprises filling a container with an adsorbent such as activated carbon, zeolite, or silica gel, and then adsorbing and storing a natural gas or the like in the container has been proposed in order to store a large amount of a fuel gas such as natural gas under a relatively low pressure.
For example, in Japanese Patent Application Laid-Open No. 258961/1986, there is disclosed the application of such a storage method for use in automobiles.
However, in this conventional method of storing natural gas by the use of the adsorbent, while a large amount of gas can be adsorbed when pure methane is stored, when a natural gas such as Japanese 13A town gas (the main component of which is methane and which also contains additional hydrocarbons such as ethane, propane, butane) is adsorbed and stored, storage density (V/V0) remarkably decreases. This phenomenon is believed to occur because higher carbon components such as propane and butane contained in the natural gas are liquefied in the pores of the adsorbent and clog these pores, thereby impeding the adsorption of methane.
In an example shown in FIG. 15 components of a natural gas enter a pore 52 in an adsorbent 50, such as activated carbon, and are adsorbed. It is intended that the diameter of the pore gradually decrease toward the inside, but, when large molecules 54, being hydrocarbons with larger particle diameters such as propane, butane, and the like enter inside of small molecules 56 of methane or ethane, the large molecules 54 are caught midway in the pore 52, where it is difficult to desorb these trapped large molecules 54. Because the large molecules 54 of propane, butane, and the like have slower molecular velocities, and stronger affinity for the wall of the adsorbent 50, the large molecules 54 are more difficult to desorb than the small molecules 56 of methane or ethane. Additionally, the pressure in the pore 52 is reduced before the adsorption of the natural gas, and, once the inside of the pore 52 is clogged with the large molecules 54, the pressure difference between the inside and the outside of the pore 52 further impedes desorption of the large molecules 54. In this manner, when the inside of the pore 52 is clogged with the large molecules 54, a space is produced at the tip end of the pore 52 because the large molecules 54 cannot advance into the innermost part of the pore 52. Because the component molecules of the natural gas are not adsorbed in this open space, the effective volume of the pore 52 is decreased, thereby decreasing the amount of gas adsorbable by the adsorbent 50.
This decrease becomes especially remarkable as the adsorption/desorption of the natural gas is repeated because additional large molecules 54 clog the pores 52 each time the adsorption/desorption of the natural gas is repeated.
Therefore, the conventional adsorption storage method as described above has a significant problem making its practical use difficult.
Activated carbon is commonly used as an adsorbent for adsorbing and storing natural gas. An improved technique for adsorbing and storing natural gas in activated carbon is disclosed in Japanese Patent Application Laid-Open No. 55067/1994.
Generally, reduction of pore diameter is known to be effective for lowering the potential of natural gas adsorbed in the pores of an adsorbent such as activated carbon and for thereby stabilizing adsorption and storage. Therefore, activated carbon of the smallest available pore diameter is commonly used. In the above-mentioned art, activated carbon with a pore diameter on the order of 5 to 25 angstroms is disclosed, and it is further described elsewhere that the pore diameter about twice the diameter of a methane molecule, that is, of about 11.6 angstroms is preferable.
When the pore diameter is reduced as in the above-described conventional activated carbon, at a pressure as low as about several atmospheres a larger amount of natural gas can be stored than when the natural gas is simply compressed. However, when the pore diameter is small, there is a problem that, even when the storage pressure is raised to increase the storage amount, the adsorption amount does not greatly increase. This is because, when the pore diameter of activated carbon is set to an extremely small value of the order of 5 to 10 angstroms, the adsorption phenomenon becomes saturated at a relatively low pressure. This saturation pressure tends to lower as the pore diameter of the activated carbon decreases.
Moreover, when the activated carbon pore diameter is reduced, it becomes difficult to desorb the natural gas adsorbed in the pores of the activated carbon, so that a step of heating the activated carbon during the desorption or another method must be employed. Therefore, when activated carbon with a small pore diameter is used, there is also a problem that the adsorbed and stored natural gas cannot readily be used.
The present invention has been developed in consideration of the above-described problems, and an object thereof is to provide an adsorption storage method of a natural gas and an adsorbent for use in the method in which, even when a practical natural gas is used, a high storage density (V/V0) can be secured.
To attain the above-described object, according to the present invention, there is provided an adsorption storage method of a natural gas which comprises the steps of separating the natural gas into a low carbon component and a high carbon component, and independently adsorbing and storing in an adsorbent the low carbon component under a high pressure and the high carbon component under a low pressure. Moreover, in the adsorption storage method of the natural gas, there are provided a first adsorption tank containing the adsorbent to adsorb and store the low carbon component, and a second adsorption tank containing the adsorbent to adsorb and store the high carbon component, the pore diameter of the adsorbent contained in the second adsorption tank being smaller than that of the adsorbent contained in the first adsorption tank, wherein the natural gas is supplied to the first adsorption tank via the second adsorption tank.
Furthermore, in the adsorption storage method of the natural gas, the second adsorption tank may be provided with cooling means.
Additionally, in the adsorption storage method of the natural gas, after the natural gas is temporarily introduced into the second adsorption tank, the pressure may be once lowered before the natural gas is introduced again.
Moreover, in the adsorption storage method of the natural gas, it may be preferable that, when the stored natural gas is desorbed and used, the gas desorbed from the first adsorption tank be removed via the second adsorption tank.
In an additional aspect of the present invention, an adsorption storage method of a natural gas comprises the steps of adsorbing a gas having a smaller molecular size than propane in the adsorbent, and adsorbing the natural gas in the adsorbent.
Additionally, in the adsorption storage method of the natural gas, the adsorbent may be heated to 20xc2x0 C. or more.
Moreover, in the adsorption storage method of the natural gas, the temperature of the adsorbent may be lowered as the natural gas is adsorbed.
An adsorption storage method of a natural gas according to a further aspect of the present invention is characterized in that, when the natural gas is adsorbed and stored in an adsorbent, the natural gas is adsorbed as it is caused to flow through a gap between the adsorbents.
Additionally, an adsorption storage method of a natural gas by adsorption to an adsorbent may comprise steps of first adsorbing a gas with a smaller molecular size than that of propane into the adsorbent; and subsequently adsorbing the natural gas to the adsorbent.
Moreover, in the adsorption storage method of the natural gas, steps of desorbing the natural gas from the adsorbent under a pressure not greater than the pressure under which the gas having a smaller molecular size than propane was adsorbed, and then again adsorbing only the natural gas may preferably be included.
Moreover, in the adsorption storage method of the natural gas, the gas may be methane or ethane with a high purity.
Furthermore, an adsorbent for use in adsorption and storage of a natural gas may comprise activated carbon subjected to a pressure reducing treatment during a high temperature activating treatment.
Additionally, in the adsorbent, the activated carbon may be treated with an activating treatment agent to which lithium bromide or lithium chloride is added.
Moreover, an adsorbent for use in adsorption and storage of a natural gas may comprise activated carbon which is washed in an organic solvent, and subsequently calcined in an inactive atmosphere or a hydrogen atmosphere in an activating treatment.
Furthermore, a normal paraffin may be adsorbed before the natural gas is adsorbed. Additionally, a side chain paraffin may be separated/removed from the natural gas before the natural gas is adsorbed.
Still further, before the natural gas is adsorbed, the natural gas may be separated into a first component containing no side chain paraffin and a second component containing the side chain paraffin, the first component adsorbed, and then the second component be adsorbed.
Furthermore, in an adsorbent for use in adsorption and storage of a natural gas, the density of pores with pore diameters of 10 angstroms or less is 0.1 cc/g or less.
Additionally, in the adsorbent, a preferable pore diameter distribution peak may be in a range of 12 to 35 angstroms.
Moreover, in the adsorbent, the pore surfaces may be coated with a metal selected from the group consisting of Cu, Fe, Ag, Au, Ir and W.
Furthermore, in the adsorbent, the amount of the metal coated on the surfaces of the pores may preferably be in a range of 5 to 50 wt %.